Hart was baptized at the Maidenhead Meetinghouse (now the Presbyterian Church of Lawrenceville) on December 31, 1713.[4] He was the son of Captain Edward Hart, a farmer, public assessor, Justice of the Peace, and leader of a local militia unit during the French and Indian War, and grandson of John Hart, a carpenter who came to Hopewell from Newtown, Long Island.

In 1741, John Hart married Deborah Scudder (1721–1776). The couple would have thirteen children: Sarah, Jesse, Martha, Nathaniel, John, Susanna, Mary, Abigail, Edward, Scudder, an infant daughter, Daniel, and Deborah, of whom only Daniel and Deborah were still minor children at the time of John Hart's death in 1779. Deborah Hart predeceased her husband, dying October 28, 1776. In 1747 he donated a piece of land in his front meadow to local Baptists who had been seeking a place to build a church. The location was known for some time thereafter as the Old Baptist Meeting House.[3] John Hart is buried there.

When New Jersey formed a revolutionary assembly (or provincial congress) in 1776, he was elected to it and served as its Vice President. Prior to June 1776, the New Jersey delegation in the First Continental Congress was opposed to independence. As a result, the entire delegation was replaced, and Hart was one of those selected for the Second Continental Congress. He joined in time to vote for and sign the Declaration of Independence.

He served until August of that year, then was elected Speaker of the newly formed New Jersey General Assembly. He would later take on additional duties as Treasurer of the Council of Safety (which was given "extraordinary and summary powers" to carry out affairs of the state during emergencies), President of the Joint Meetings of the New Jersey Congress, and Commissioner of the State Loan Office. On June 22, 1778, he invited the American army to encamp on his farm. Washington had lunch with him, then had his famous Council of War at the nearby Hunt House. Twelve thousand men camped on his fields-during the growing season. After resting and preparing for battle the troops left on the 24th.

In December 1776, the British advance into New Jersey reached Hunterdon County. A marked man due to his status as Speaker of the Assembly, Hart was obliged to escape and hide for a short time in the nearby Sourland Mountains. His farm was raided by British and Hessian troops, who damaged but did not destroy the property. The Continentals' capture of Trenton on December 26 allowed Hart to return home.

Prior to the Battle of Monmouth, Hart invited Gen. George Washington and the Continental Army to make camp on his farm, and his offer was accepted. From June 22–24, 1778, 12,000 men occupied his fields, and on at least one occasion Gen. Washington dined with their host.

On November 7, 1778, John Hart returned to Hopewell from the Assembly in Trenton. Two days later, he indicated that he was too ill with "gravel" (kidney stones) to return. He continued to suffer from the painful affliction for more than six months until his death on May 11, 1779.

The following obituary for John Hart appeared on May 19, 1779:

On Tuesday the 11th instant, departed this life at his seat in Hopewell, JOHN HART, Esq. the Representative in General Assembly for the county of Hunterdon, and late Speaker of that House. He had served in the Assembly for many years under the former government, taken an early and active part in the present revolution, and continued to the day he was seized with his last illness to discharge the duties of a faithful and upright patriot in the service of his country in general and the county he represented in particular. The universal approbation of his character and conduct among all ranks of people, is the best testimony of his worth, and as it must make his death regretted and lamented, will ensure lasting respect to his memory.